Variability of solar-induced (natural) fluorescence and chlorophyll were measured on scales of hours to weeks in the upper layer of a cyclonic eddy located south of the Antarctic Polar Front using a free-floating drifter. The fluorescence signal was analyzed both in terms of chlorophyll concentration and as an indicator of energy distribution in the photosynthetic apparatus. Long-term trends in fluorescence parallel changes in chlorophyll concentration. Considering a significant positive correlation between fluorescence and the relative depth of the eddy upper layer we hypothesize that the observed short-term variations in natural fluorescence are a physiological response of phytoplankton to changes in the supply of limiting nutrients. This interpretation is consistent with the Southern Ocean iron limitation hypothesis.¿ 1997 American Geophysical Union